probably at the local goodwill store since you are not willing to spend a decent amount for a good quality racquet. don't mean to be an ___. but if that's what you can afford. then make the best of it. especially if you are just starting out. you are probably better off investing that money on lessons.

I agree with junbug, $30 wont get you anything but a cheap aluminum frame from your local sporting goods store or a very old (and strained out) frame from goodwill. A performance frame costs $150-$250. As you get better you will probably want to get one and you will have to learn about racquet specs to know which ones suit your style of play and which ones to seriously look at or demo. As a beginner you probably want an oversize frame to help you with off-center shots. As you get better learn the specs. There are some good spec tables on www.NothingButTennis.com, look in the FAQ pages i think. Now that i understand all the specs, i can buy a frame without even demoing it.

I hate to say this,
but if you want to get a decent racket then your going to have to spend at least $50+
If your looking to get a cheap, non-decent racket you could go for the $30 dollar Dunlop's at K-mart but they are absolutely pathetic, in my opinion.
I'd suggest buying a Wilson. Currently, I play with the Adidas Barricade and 2 other Wilson's and I think the Wilson's overpower every brand I've played with.
Expensive but the best you can fine.